1. Field of the Invention
The present invention relates to a charged particle beam irradiation system and a method of extracting a charged particle beam. More specifically, the invention relates to a charged particle beam irradiation system and a method of extracting a charged particle beam that can be suitably applied to a material irradiation apparatus for irradiating a material with a charged particle beam, a food irradiation apparatus for irradiating food with a charged particle beam, and a particle beam therapy system for irradiating an affected part of a body with a charged particle beam, such as a proton or carbon ion beam.
2. Description of the Related Art
There is a known method of cancer treatment in which the affected part of the body of a patient is irradiated with a charged particle beam (ion beam), such as a proton or carbon ion beam. An ion beam irradiation system includes an ion beam generator, a beam transport system, and an irradiation device. The ion beam generator accelerates an ion beam circulating around an orbit to a predetermined energy. An ion beam that is accelerated to the predetermined energy is transported to the irradiation device via the beam transport system and is emitted to an object.
The ion beam generator includes a circular accelerator, such as a synchrotron or a cyclotron. Japanese Patent No. 2,596,292 describes, as an ion beam generator, a circulating unit for circulating an ion beam along an orbit, a radio frequency (RF) applying device for increasing the amplitude of betatron oscillation of an ion beam inside a separatrix in phase space, and an electrostatic deflector for extracting an ion beam from the orbit. The RF applying device increases the amplitude of betatron oscillation of an ion beam stably orbiting inside the synchrotron by applying an RF magnetic field or an RF electric field when the ion beam is accelerated to a predetermined energy inside the synchrotron. An ion beam whose amplitude of betatron oscillation is increased transits outside the separatrix and is extracted from the synchrotron to the beam transport system.
In a particle beam therapy system, an irradiation device extracts an ion beam and emits this ion beam into a patient's body. The ion beam has a physical characteristic in which it deposits most of its energy at the end of its path (i.e., at its Bragg peak). The position in the patient's body where the Bragg peak is generated depends on the energy of the ion beam.
Normally, an affected part of the body of the patient has a certain thickness along the depth direction (i.e., beam progression direction) from the surface of the body. To irradiate the entire thickness of the affected part in the depth direction, the energy of the ion beam has to be controlled to generate a spread-out Bragg peak (SOBP) that is spread out uniformly in the depth direction.
From such a standpoint, a charged particle beam irradiation system including an irradiation device provided with a range modulation wheel (RMW) has been proposed (refer to “Review of Scientific Instruments,” Vol. 64, No. 8, August 1993, p. 2,077, FIG. 30). The RMW includes a plurality of blades disposed along the circumferential direction. The longitudinal cross-section of the blades is wedge-shaped such that the thickness of the blades gradually increases or decreases in the axial direction. The RMW is provided in the beam path inside the irradiation device and rotates on a plane orthogonal to the beam path. As the charged particle beam passes through the rotating RMW, the position where the Bragg peak is generated in the patient's body changes periodically. As a result, based on time integration, a SOBP that is relatively spread out from near the surface of the body to inside the body is generated.
JP, A 2004-529483 describes a technology for adjusting the intensity of a proton beam by changing the electrical current supplied to the ion source. More specifically, according to JP, A 2004-529483, to obtain a beam with a predetermined intensity by using a cyclotron, the intensity of an extracted beam is actually measured, and the electrical current supplied to the ion source is control on the basis of the measurement results.
A charged particle beam irradiation apparatus described in JP, A 11-408 includes a pair of filters for absorbing energy. The filters have spiral sections whose thickness in the shaft direction changes in a spiral manner around the center of the shaft. The filters are disposed in an overlapping manner in the beam path and rotate at a constant speed in opposite directions. By turning on and off the ion beam source according to the rotational angles of the filters, the thickness of the overlapping spiral sections where the beam passes through is controlled to shift the depth reached by the beam in the object being irradiated. In this way, the dose distribution in the depth direction of the object can be controlled.